This study proposed a prediction formula for the torsional strength enabling to reflect the tensile strength of ultra high performance concrete (UHPC) beams based upon the thin-walled tube theory. The remarkable ductile behavior of UHPC can also be attributed to the steel fiber reinforcement. This feature must be considered to provide rational explanation of the torsional behavior of UHPC structures. In this study, the proposed torsional design adopts a modified thin-walled tube theory so as to consider the tensile behavior of UHPC. And torsion test was conducted on thinwalled UHPC box beams to validate the proposed formula through comparison of the predicted torsional strength with the experimental results. The comparison of the predicted values of the cracking torque and torsional moment resistance with those observed in the torsional test of UHPC verified the validity of the design method. The contribution of the steel fibers to the torsional strength and cracking load was larger than that of the stirrups, but the stirrups appeared to contribute additionally to the torsional ductility. Accordingly, it is recommended that design should exploit effectively the contribution of the steel fiber rather than arrange a larger number of stirrups in UHPC structures subjected to torsion.
This paper investigates experimentally the feasibility of estimating the stress in the prestressing tendons of prestressed concrete bridges using the magnetic field induced by an electromagnet and the Villari effect in which the magnetic susceptibility or permeability of a ferromagnetic material changes when subjected to a mechanical stress. The test results show the good linearity between the stress in the prestressing tendon and the induced magnetic flux density within the practical stress range of the tendons. In addition, the induced magnetic flux density in the tendon appears to depend on the intensity of the electromagnet and the distance between the electromagnet and the tendon regardless of the concrete cover. Accordingly, although further studies are needed for practical applications, the stress in the prestressing tendon of a prestressed concrete bridge can be estimated by measuring the induced magnetic flux density generated in the tendon and by using the linear relationship between this induced magnetic flux density and the stress in the prestressing tendon.
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